1. Field of the Invention
The present invention generally relates to small propulsion systems for maneuvering spacecraft and, more particularly, is concerned with an electrothermal arcjet thruster employing any one of several different features for improving arc attachment and thereby achieving enhanced efficiency.
2. Description of the Prior Art
As conventionally known, an electrothermal arcjet thruster converts electrical energy to thermal energy by heat transfer from an arc discharge to a flowing propellant and from thermalenergy to directed kinetic energy by expansion of the heated propellant through a nozzle. For an explanation from an historical perspective of arcjet thruster construction and operation and the problems associated with this type of electrothermal propulsion, attention is directed to the following publications: "Arcjet Thruster for Space Propulsion" by L. E.. Wallner and J. Czika, Jr., NASA Tech Note D-2868, June 1965; "The Arc H Engine" by F. G.. Penzig, AD 671501, Holloman Air Force Base, March 1966; and "Physics of Electric Propulsion" by R. G.. Jahn, McGraw-Hill Book Company, 1968. Attention is also directed to U.S. Pat. No. 4,548,033 to G. L.. Cann.
Most electrothermal arcjet thrusters have as common features an anode in the form of a nozzle body and a cathode in the form of a cylindrical rod with a conical tip. The nozzle body has an arc chamber defined by a constrictor in a rearward portion of the body and a nozzle in a forward portion thereof. The cathode rod is aligned on the longitudinal axis of the nozzle body with its conical tip extending into the upstream end of the arc chamber in spaced relation to the constrictor so as to defined a gap therebetween.
Electrothermal arcjet thrusters currently being developed are limited in their efficiency principally by a loss mechanism called frozen flow losses. Frozen flow losses include ionization, disassociation, and deposition of energy into excited molecular states. Frozen flow losses occur when the propellant gas is heated to very high temperatures by close contact with an electric arc and then exhausted out a nozzle. In standard arcjet thrusters, insufficient time in high pressure regions is allowed to recombine the ions or disassociated molecules or to relax the excited states. Energy locked into these processes is, therefore, lost and unavailable for thrust. In addition to the frozen flow losses in the standard constricted arc geometry, standard constricted arcjets are not tolerant to large fluctuations in mass flow because the arc is "blown" into regions where heat transfer and the conversion of thermal energy into kinetic energy is inefficient.
Consequently, a need exists for a fresh approach to improvement of arc attachment in an arcjet thruster in a way which will enhance the efficiency thereof.